The present invention relates to a film for protecting an image for protecting an image formed by a recording system such as an ink jet system and a method for producing a recorded material using the same.
An ink jet system is a printing system to carry out printing by discharging droplets of ink from a minute nozzle of a recording head to adhere the same to a recording face of a recording medium such as paper. Recently, a thermal transfer type overcoat system is known in which, by using a film for protecting an image having a constitution of laminating a thermoplastic resin layer on a support, the resin layer is heated and pressure bonded on the recorded face of a recording medium on which an image has been formed by an ink jet system, subsequently the support is detached from the resin layer to provide a protective layer constituted of the resin layer, in order to improve an image fastness and the like (for example, refer to JP-A-2000-233474).
Generally, in transferring and forming a protective layer by an overcoat system treatment (overcoat system treatment includes a detaching process-free system by using a film for protecting an image with an adhesive layer in addition to the thermal transfer type having a detaching process of a support) including the thermal transfer type, it is usual to employ a process allowing a thermoplastic resin layer to transfer thermally from a support to a recording medium by using a heating and pressure bonding apparatus. However, consideration of damage of a printed ink-accepting layer with heat and degradation of a recorded image due to it gives no other choice of a limited temperature of heat and period of time that can be added by the heating and pressure bonding apparatus. Therefore, sometimes adhesiveness between a recording medium and a thermoplastic resin layer (protective layer) is not sufficient in the related method.
Further, in recent years, high speed ink absorption properties have been required for an ink jet recording medium along with speed up of printing by an ink jet system and compositional ratio of a porous inorganic particle such as amorphous silica to a binder in the ink-accepting layer thereof demonstrates an upward trend. However, surface of such the ink-accepting layer forms irregularities to give poor smoothness and adhesiveness of a thermoplastic resin layer to the surface tends to decrease significantly. This is due to the fact that sufficient melting and penetration of the thermoplastic resin layer into concave portions tends to occur difficultly.
As described above, a thermal transfer type overcoat system using a related film for protecting an image is forced to carry out thermal transfer in a mild heating condition that does not induce degradation of a recorded image by heat. Therefore, adhesiveness of a thermoplastic resin layer to a recording medium, especially to an ink jet recording medium having an ink-accepting layer containing a porous inorganic particle is quite inadequate, resulting in such a problem that a part of the resin layer is lifted from the recorded face or, in a worse case, is detached with a support during a process for detaching a support (decrease in detachability of a support). The lift of a resin layer from the recorded face becomes especially problematic when an image is formed by using a pigment ink. That is, when a pigment ink is driven into a recording face, most of coloring material (pigment) do not penetrate into a recording medium and attach to the recording face. Therefore, the lift of the resin layer induces decreasing in a image density or transferring the coloring material to the lifted resin layer side, resulting in generation of bleeding or stripping of the image.
Therefore, as a technique for achieving an excellent adhesiveness with a relatively mild heating condition that does not generate degradation of a recorded image with heat, there may be a method to lower the glass transition temperature (Tg) of a resin constituting a thermoplastic resin layer in a film for protecting an image. However, when the glass transition temperature Tg of a thermoplastic resin layer to become a protective layer by being transferred to a recording medium is lowered to an extent that can achieve the purpose, scratch resistance (abrasion resistance) of the thermoplastic resin layer decreases to generate such a problem that surface of the protective layer tends to be suffered by scratches and the like. In addition, use of a thermoplastic resin having a low glass transition temperature Tg makes the surface of the protective layer sticky at ordinary temperature to generate another problem of decrease in blocking resistance.